The present invention relates to an electrophotographic recording apparatus or printer, more particularly to a method of controlling the transfer voltage in an electrophotographic printer.
An electrophotographic printer transfers a toner image from a photosensitive drum to a recording medium such as paper by applying a voltage to a transfer roller as the recording medium passes between the transfer roller and the photosensitive drum. The applied voltage, referred to as the transfer voltage, creates an electric field in the space between the photosensitive drum and transfer roller. The electric field attracts toner particles from the photosensitive drum to the recording medium.
The optimum transfer voltage depends in part on the electrical resistance between the transfer roller shaft and the surface of the transfer roller. This resistance varies with ambient temperature and humidity conditions, and with aging changes in the transfer roller. The resistance may also vary when measured at different points on the roller surface, due to physical irregularities in the roller composition. In addition, the capacitance of the transfer roller needs to be fully charged before the transfer process begins, and the charging rate varies according to the roller resistance. For these reasons, printing defects may occur if the printer's control system simply supplies a fixed transfer voltage to the transfer roller for a fixed time.
U.S. Pat. No. 5,682,575 discloses a method of adjusting the transfer voltage according to resistance measurements made both before and after the recording medium enters the space between the photosensitive drum and transfer roller, but this method does not allow for variations in charging time, or variations in resistance as the roller rotates.